1. Field of Invention
The present invention relates to improved gypsum wallboard and to materials and processes for making such products.
2. Background
Gypsum wallboard is conventionally made by depositing an aqueous slurry of calcined gypsum (xe2x80x9cgypsum slurryxe2x80x9d) between large sheets of paper or other material and allowing the slurry to dry. Calcined gypsum is composed of calcium sulfate hemihydrate (CaSO4.xc2xdH2O) which rehydrates to gypsum (CaSO4.2H2O) during the drying process. See Kirk Othmer, Encyclopedia of Chemical Technology, Second edition, 1970, Vol. 21, Pages 621-624, the disclosure of which is also incorporated herein by reference.
In order to achieve sufficient strength, traditional, commercial wallboard has been made with a density of about 1700 pounds (xcx9c772 kg.) per thousand square feet of xc2xd inch thick board. Although it would be desirable to reduce this density and hence overall board weight, previous attempts have met with limited success, primarily due to loss of strength.
U.S. Pat. No. 5,879,825 to Burke et al., the disclosure of which is also incorporated herein by reference, describes an approach for reducing gypsum wallboard density without sacrificing strength by including in the gypsum core an acrylic latex having a particular combination of properties. Acrylic latexes are expensive, and therefore commercially unattractive. Therefore, it is desirable to develop an alternate and less expensive approach to accomplishing this objective.
In accordance with the present invention, it has been discovered that certain styrene butadiene latexes, modified to be substantially stable against divalent ions, can also reduce gypsum wallboard density without sacrificing strength. Because these latexes are generally less expensive to manufacture than acrylic latexes, it has also been found that commercial use of these latexes is feasible.
Accordingly, the present invention provides a new composition for making gypsum products comprising water, calcium sulfate hemihydrate and a styrene butadiene polymer latex substantially stable against divalent ions in which the styrene butadiene polymer includes at least 0.25 wt. % of an ionic monomer. In addition, the present invention also provides a new process for making gypsum wallboard from this composition as well as the wallboard so made. Preferably, the styrene butadiene polymer of the latex used in the present invention includes copolymerized sodium 2-acrylamido-2-methyl propanesulfonic acid salt, known industrially as xe2x80x9csodium AMPS.xe2x80x9d